hashsum: don't exit early on io errors

[Qelxiros]

closes #7906

exact GNU behavior is as follows (the issue wasn't very clear): if an IO error is encountered (I tested with file not found and permission denied), the binary name, file name, and error are printed to stderr and the program continues. If any errors are found, the program exits with status code 1.

[github-actions[bot]]

GNU testsuite comparison:

Skip an intermittent issue tests/misc/stdbuf (fails in this run but passes in the 'main' branch)

[github-actions[bot]]

GNU testsuite comparison:

Skip an intermittent issue tests/misc/tee (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/timeout/timeout (fails in this run but passes in the 'main' branch)

[github-actions[bot]]

GNU testsuite comparison:

Skip an intermittent issue tests/misc/tee (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/timeout/timeout (fails in this run but passes in the 'main' branch)

[RenjiSann]

Thanks !